**6. From (CrAl)N or (AlCr)N to Oxynitride and Oxide Coatings**

The special features of the MeN1−*x*O*<sup>x</sup>* coating arise from the addition of oxygen to MeN, which results in coating properties between those of metal nitrides, MeN, and those of the insulating oxides, MeO [165]. Tuning the oxygen/nitrogen ratio allows the physical, mechanical and tribological characteristics to be tailored. For AlCr-based coatings, both oxynitrides as well as pure oxide coatings of AlCr are of interest. It was shown that oxynitride and oxide coatings (alpha and cubic crystal modifications) have a potential for use in various tribological applications and for oxidation protection [166–170], and moreover, even for use in solar absorbers [171]. Both vacuum arc deposition and various sputtering methods, including HiPIMS processes and hybrid processes (HiPIMS plus DC magnetron sputtering), were applied to investigate coatings of the types (AlCr)ON and (AlCr)O [23,172–175].

In the context of oxynitrides, it should be noted that in many industrialised nitride deposition processes, residual amounts of oxygen (up to several at.%) are incorporated [111, 122,146,172,175,176]. The effects of substrate rotation additionally influence details of the structure due to the changing position in the chamber relative to the source [132]. These effects occur in industrial coating systems and are frequently not discussed. However, for example, it was observed that a small amount of oxygen can decrease the hardness of nanocomposites significantly [177]. A detailed discussion of these effects is beyond the scope of this paper. However, actively controlling oxygen content in the coatings is a potent modifier of structure and properties, as will be exemplified in this section.

Oxynitride coatings and oxide coatings have often been investigated in one and the same paper. Figure 23 shows a schematic representation of the phase evolution in (CrAl)NO and (AlCr)NO systems dependent on Cr/(Al + Cr) and O/(O + N) at a typical deposition temperature of about 500–600 ◦C based on results from [166,167,172,173,178]. The differentiation between Cr-rich and Al-rich coating types is ignored here in the interest of facilitating visual perception.

**Figure 23.** Schematic representation of phase evolution in the (AlCr)NO system dependent on Cr/(Al + Cr) and O/(O + N) at a deposition temperature in the range of about 500–600 ◦C, c = cubic structure, amo. = amorphous structure.

Oxynitrides are formed up to an estimated oxygen content of about 40 at.%. Above this threshold, a mixed phase structure is formed, consisting of oxynitrides and oxide phases as well as sometimes a minor fraction of amorphous phases as well. At high oxygen contents, oxides are formed. A cubic oxide structure (gamma or fcc) grows at a low Cr/(Cr + Al) content, whereas a hexagonal oxide structure (alpha) is predominant at a higher Cr/(Cr + Al) content. The formation of an oxide coating structure with a dominating fcc phase has also been reported a number of times [166,179,180]. The lowest reported value to obtain a pure corundum phase was Cr/(Cr + Al) = 0.3 [178]. There are also several reports on the existence of minor amorphous phases [175,178,179].

Selected results will be presented in more detail for the two deposition methods, arc and sputtering.

### *6.1. Arc Evaporation: Oxynitride and Oxide Coatings*

One of the earliest approaches to synthesising oxides by arc evaporation is reported in a patent application (1992) from Schulz and Bergmann for the deposition of α-Al2O3 type coatings by DC arc evaporation at a lower temperature than in CVD (Chemical Vapor Deposition) processes, when the Al is doped using Cr or other selected elements [22]. The field, however, remained relatively unexplored well into the 2000s.

Investigations of coatings containing Al, Cr, O and N deposited by the vacuum arc method using different cathode compositions (Al/Cr) and different reactive gas compositions (N2/O2) are shown in Figure 24.

Upon increasing the oxygen fraction in the gas flow, first an oxynitride is formed, followed by an N-doped oxide and then a pure oxide [166]. Different phases are formed depending on the oxygen content in the reactive gas and the metal composition of the cathodes. In oxynitride coatings, the phase composition is first predominantly cubic, then a mixed zone is observable, comprising the solid solutions fcc (Cr,Al)N, and nitrogen-doped cubic (Cr,Al)2O3 (gamma or fcc) or, finally, α (Cr,Al)2O3 at a pure oxygen flow. The oxide structures formed were either cubic or α (Cr,Al)2O3. Figure 25 shows the tendencies for the formation of either cubic or α (Cr,Al)2O3.

**Figure 24.** (**a**) Al + Cr to N + O ratio (**b**) N and O content in Al-Cr-O-N coatings for different N2/O2 reactive gas mixtures, redrawn after [166], original © Elsevier.

**Figure 25.** Schematic representation of the influence the coating composition has on the trend of phase formation in (AlCr)2O3, redrawn after [166], original © Elsevier.

A low level of O and a high Al content (e.g., using cathodes of Al66Cr34) in the coatings results in the formation of cubic (Cr,Al)2O3 layers. Higher levels of O and Cr (e.g., using cathodes of Cr75Al25) in the coatings increase the probability of the deposition of α (Cr,Al)2O3 layers.

The Cr-rich (CrAl)O coatings show the highest hardness values of 31–34 GPa, while the Al-rich coatings have lower values of 24–28 GPa. Metal cutting tests using cemented carbide inserts in turning operations showed good wear properties for mainly oxygen-rich coatings. These results were better than with the presence of the corundum phase of the oxide. Fcc-(Cr,Al)2O3 dominated coatings have also been shown to have wear properties similar to those of α-(Cr,Al)2O3 coatings [166].

In conclusion, the ideal structure is a corundum-type solid solution, preferably with a high Al-content (>60 at.% on the metal sublattice). However, at a high Al-content, the tendency towards the formation of dual-phase compositions containing metastable phase fractions has been observed, which may negatively influence the mechanical properties and performance. The authors concluded that the cathodic arc evaporation of (Al,Cr)2O3-based coatings is very complex when the microstructure of the arc cathode used is included as a factor [174].

In combination with (AlCr)N, a small addition of O using Al70Cr30 in the top layer of a double layer coating ((AlCr)N/(AlCr)NO) has been reported to increase the service life of tools in milling applications [181]. The effect of the oxygen content in doped AlCr based coatings, e.g., (AlCrSi)N, has also been investigated [132].

It should be mentioned that a special pulsed arc process, the P3eTM, was developed to prevent oxide contaminations of the cathodes and increase the reactivity of the metal vapour and reactive gas [23].
